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Development of state-of-the-art time-resolved optical and electronic spectroscopy

Construction of time- and angle-resolved photoemission spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) is able to directly measure the energy and momentum of an electron, dubbed as band structure, in solid-state materials. The principle is based on the celebrated Einstein photoelectric effect, in which a photon with sufficient energy can eject electrons from the material. Using femtosecond lasers, the method can be extended to a time-resolved version in which not only can unoccupied excited electronic states be accessed but also the time evolution of the electrons accompanied by potential collective modes can be traced. Honglie has been working on a laser-based ARPES which can operate at an unprecedentedly low temperature (0.7 K) with high energy resolution (5 meV) in the first three years of his Ph.D. Meanwhile, he also designed and constructed a 4th harmonic generation setup to generate 6.2 eV probe based on Ti:sapphire laser (1.55 eV) so that time-resolved ARPES function is realized. Further upgrade is undergoing.

Construction of time-resolved THz reflection and emission spectroscopy and intense THz pump

Time-domain THz spectroscopy (TDTS) allows for a phase-sensitive measurement of both real and imaginary optical conductivity as low as 1-24meV. Honglie has been working on developing a reflection-based TDTS setup and further extending to a time-resolved version by incorporating a near-infrared pump. A ZnTe/GaP crystal is used to generate the THz probe through optical rectification of the 800 nm pulses from the Ti:sapphire laser, and another ZnTe/GaP crystal detects the reflected THz pulse via electro-optic sampling. Removing the generation crystal the setup naturally transforms into a THz emission setup, which can be served as a DC-limit second harmonic generation setup. Employing a rotation stage, we are able to conduct THz emission polarimetry measurement to clarify the symmetry of the electron near the Fermi levels. Currently, Honglie is working on upgrading the setup to accommodate an intense THz pump generated by organic crystal. This can extensively expand the pump energy down to 1-10 meV where abundant charge, magnetic, orbital, and lattice excitations reside and can thus be resonantly excited.

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